Name the ~= operator when introducing it (#306)

Fixes: swiftlang/swift-book#304

Author: amartini51

TSPL.docc/ReferenceManual/Patterns.md

@@ -411,7 +411,7 @@ case labels.
 
 The expression represented by the expression pattern
 is compared with the value of an input expression
-using the Swift standard library `~=` operator.
+using the pattern-matching operator (`~=`) from the Swift standard library.
 The matches succeeds
 if the `~=` operator returns `true`. By default, the `~=` operator compares
 two values of the same type using the `==` operator.