[eigen] Small documentation optimizations (tiny test patch)

[Tim Holy <holy@xxxxxxxxx>]

Hello,

I've been learning Eigen recently. I may be able to help in small ways, mostly 
with documentation (I am not ready to try to help with core code yet).

I'm not sure about the proper way to submit patches, but I've attached a tiny 
test commit (fixes one misspelling). I have more documentation changes I can 
contribute; I'm fine with the idea of just emailing patches, but if this is not 
how you'd prefer to receive them, please let me know now.

Best,
--Tim

# HG changeset patch
# User Tim Holy <holy@xxxxxxxxx>
# Date 1308512359 18000
# Node ID fb75737564e95fa19d9bab26beeb34d7247280b8
# Parent  d293654f07e4510c34be0a08a182ee8b635c983f
A first tiny test commit: fix a spelling error in the documentation.

diff --git a/doc/C03_TutorialArrayClass.dox b/doc/C03_TutorialArrayClass.dox
--- a/doc/C03_TutorialArrayClass.dox
+++ b/doc/C03_TutorialArrayClass.dox
@@ -150,17 +150,17 @@ access to all operations regardless of t
 \link MatrixBase Matrix expressions \endlink have an \link MatrixBase::array() .array() \endlink method that
 'converts' them into \link ArrayBase array expressions\endlink, so that coefficient-wise operations
 can be applied easily. Conversely, \link ArrayBase array expressions \endlink
 have a \link ArrayBase::matrix() .matrix() \endlink method. As with all Eigen expression abstractions,
 this doesn't have any runtime cost (provided that you let your compiler optimize).
 Both \link MatrixBase::array() .array() \endlink and \link ArrayBase::matrix() .matrix() \endlink 
 can be used as rvalues and as lvalues.
 
-Mixing matrices and arrays in an expression is forbidden with Eigen. For instance, you cannot add a amtrix and
+Mixing matrices and arrays in an expression is forbidden with Eigen. For instance, you cannot add a matrix and
 array directly; the operands of a \c + operator should either both be matrices or both be arrays. However,
 it is easy to convert from one to the other with \link MatrixBase::array() .array() \endlink and 
 \link ArrayBase::matrix() .matrix()\endlink. The exception to this rule is the assignment operator: it is
 allowed to assign a matrix expression to an array variable, or to assign an array expression to a matrix
 variable.
 
 The following example shows how to use array operations on a Matrix object by employing the 
 \link MatrixBase::array() .array() \endlink method. For example, the statement