Rewrite an ambiguous abstract.

The words "structure" and "groups" can both be read as either a noun or
a verb, which makes this sentence hard to parse.  This fact that we use
"structure" as a noun a lot throughout the book to refer to structs
exacerbates that misreading.

Fixes rdar://103188927

Author: amartini51

Sources/TSPL/TSPL.docc/ReferenceManual/Statements.md

@@ -1,6 +1,6 @@
 # Statements
 
-Structure groups of expressions and control their behavior.
+Group expressions and control the flow of execution.
 
 In Swift, there are three kinds of statements: simple statements, compiler control statements,
 and control flow statements.