Fix a typo in the gMock sample code for Defining a Custom Matcher Class.

EXPECT_CALL doesn't seem to make much sense here. I think the intent was to use EXPECT_THAT instead.

PiperOrigin-RevId: 495427663
Change-Id: I39a16ea37282729d34017c6bcd331a60cedf1fbe

Author: Abseil Team

docs/gmock_cook_book.md

@@ -1345,7 +1345,7 @@ class BarPlusBazEqMatcher {
 
 ...
   Foo foo;
-  EXPECT_CALL(foo, BarPlusBazEq(5))...;
+  EXPECT_THAT(foo, BarPlusBazEq(5))...;
 ```
 
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