better description for type signatures

Co-authored-by: Felicity Chapman <felicitymay@github.com>

Author: ginsbach

docs/codeql/ql-language-reference/signatures.rst

@@ -39,7 +39,7 @@ For example:
 Type signatures
 ===============
 
-Type signatures declare module parameters that are to be substituted with types at module instantiation.
+Type signatures declare module parameters that will be substituted with types when the module is instantiated.
 Type signature are the simplest category of signatures, as the only thing they allow is the specification of supertypes.
 
 The substitution of type signatures relies on structural typing. That is, types do not have to be explicitly defined as