Merge pull request #1332 from Mat-Ge/Typos

Three typos

doc/en/Authoring/Inputs/index.md

@@ -61,7 +61,7 @@ Currently STACK supports the following kinds of inputs.  These have a variety of
 Some of the special blocks provide interactions which can be linked to inputs.  These include
 
 1. **JSXGraph** diagrams.
-2, **GeoGebra** diagrams.
+2. **GeoGebra** diagrams.
 3. **Drag and drop** problems.
 
 #### True/False inputs ####

doc/en/Authoring/Potential_response_trees.md

@@ -84,7 +84,7 @@ The outcomes of score, penalty, feedback and note are always produced.  Normally
 
 There are rare, but important, situations where we need a purely formative PRT.
 
-E.g. Imagine a questions with inputs A, and B.
+E.g. Imagine a question with inputs A and B.
 
 1. Input A has a dedicated PRT to establish if it is correct.
 2. Input B has a dedicated PRT to establish if it is correct.

doc/en/Authoring/Variables.md

@@ -58,7 +58,7 @@ If the following commands appear within the question variables they will be avai
 
 This collection of special variables are called "context variables".
 
-STACK has a special constant `%_stack_preamble_end`.  Any variables _before_ this constant will be included within the context variables.  This enables you to define functions, e.g. to use with `textput`.  
+STACK has a special constant `%_stack_preamble_end`.  Any variables _before_ this constant will be included within the context variables.  This enables you to define functions, e.g. to use with `texput`.  
 
 Note, that students are not permitted to use any variable name defined by the teacher in the question variables.  This includes both the context variables, and the regular remaining question variables.  It is not possible to define variables which a student can then use.  Students _can_ use function names defined in the preamble. e.g. you can put `vec(ex):=stackvector(ex);` into the preamble.