test: fix "immutability" spelling

Author: MojtabaHs

FirebasePerformance/Tests/Unit/FIRPerformanceTest.m

@@ -118,7 +118,7 @@ - (void)testReadingAttributesFromProperty {
 }
 
 /** Validates if attributes property is immutable. */
-- (void)testImmutablityOfAttributesProperty {
+- (void)testImmutabilityOfAttributesProperty {
   [self.performance setValue:@"bar" forAttribute:@"foo"];
   NSMutableDictionary<NSString *, NSString *> *attributes =
       (NSMutableDictionary<NSString *, NSString *> *)self.performance.attributes;

FirebasePerformance/Tests/Unit/Instruments/FIRHTTPMetricTests.m

@@ -283,7 +283,7 @@ - (void)testReadingAttributesFromProperty {
 }
 
 /** Validates if attributes property is immutable. */
-- (void)testImmutablityOfAttributesProperty {
+- (void)testImmutabilityOfAttributesProperty {
   FIRHTTPMetric *metric = [[FIRHTTPMetric alloc] initWithURL:self.sampleURL
                                                   HTTPMethod:FIRHTTPMethodGET];
   [metric setValue:@"bar" forAttribute:@"foo"];

FirebasePerformance/Tests/Unit/Timer/FIRTraceTest.m

@@ -612,7 +612,7 @@ - (void)testReadingAttributesFromProperty {
 }
 
 /** Validates if attributes property is immutable. */
-- (void)testImmutablityOfAttributesProperty {
+- (void)testImmutabilityOfAttributesProperty {
   FIRTrace *trace = [[FIRTrace alloc] initWithName:@"Random"];
   [trace setValue:@"bar" forAttribute:@"foo"];
   NSMutableDictionary<NSString *, NSString *> *attributes =