Docs: improve comments of the FirebaseDatabase (#13785)

FirebaseDatabase/Tests/Unit/FTreeSortedDictionaryTests.m

@@ -173,7 +173,7 @@ - (void)testIncreasing {
 
   XCTAssertTrue([map count] == 0, @"Check if all 100 objects were removed");
   // We can't check the depth here because the map no longer contains values, so we check that it
-  // doesn't responsd to this check
+  // doesn't responds to this check
   XCTAssertTrue([map.root isMemberOfClass:[FLLRBEmptyNode class]], @"Root is an empty node");
   XCTAssertFalse([map respondsToSelector:@selector(checkMaxDepth)],
                  @"The empty node doesn't respond to this selector.");