Add a missing space to the Glossary page of TRPL

r? @steveklabnik

src/doc/trpl/glossary.md

@@ -19,7 +19,7 @@ In the example above `x` and `y` have arity 2. `z` has arity 3.
 
 When a compiler is compiling your program, it does a number of different
 things. One of the things that it does is turn the text of your program into an
-‘abstract syntax tree’, or‘AST’. This tree is a representation of the
+‘abstract syntax tree’, or ‘AST’. This tree is a representation of the
 structure of your program. For example, `2 + 3` can be turned into a tree:
 
 ```text