remove some explicit str()

docs/grading_math/matrix_grader/matrix_grader.md

@@ -237,8 +237,8 @@ For example, student enters `'[[1, 2],[3] ]'`, a matrix missing an entry in seco
 >>> try:
 ...     grader(None, student_input) # grade the input like edX would
 ... except StudentFacingError as error:
-...     str(error) # students see this error message
-"Unable to parse vector/matrix. If you're trying to enter a matrix, this is most likely caused by an unequal number of elements in each row."
+...     print(error) # students see this error message
+Unable to parse vector/matrix. If you're trying to enter a matrix, this is most likely caused by an unequal number of elements in each row.
 
 ```
 
@@ -256,8 +256,8 @@ If a student submits an answer that will raise shape-mismatch errors then an err
 >>> try:
 ...     grader(None, student_input) # grade the input like edX would
 ... except StudentFacingError as error:
-...     str(error) # students see this error message
-'Cannot add/subtract a vector of length 3 with a vector of length 2.'
+...     print(error) # students see this error message
+Cannot add/subtract a vector of length 3 with a vector of length 2.
 
 ```
 
@@ -280,15 +280,15 @@ True
 >>> try:
 ...     grader(None, student_input) # grade the input like edX would
 ... except StudentFacingError as error:
-...     str(error) # students see this error message
-'Expected answer to be a vector, but input is a vector of incorrect shape'
+...     print(error) # students see this error message
+Expected answer to be a vector, but input is a vector of incorrect shape
 
 >>> student_input = '0' # scalar; should be a vector
 >>> try:
 ...     grader(None, student_input) # grade the input like edX would
 ... except StudentFacingError as error:
-...     str(error) # students see this error message
-'Expected answer to be a vector, but input is a scalar'
+...     print(error) # students see this error message
+Expected answer to be a vector, but input is a scalar
 
 ```