[docs] stdlib guide - fix legacy naming

_sanityCheck is now called _internalInvariant

Author: bradleymackey

docs/StandardLibraryProgrammersManual.md

@@ -98,7 +98,7 @@ On these platforms, the Swift Standard Library ships as an integrated part of th
 
 #### Unwrapping Optionals
 
-Optionals can be unwrapped with `!`, which triggers a trap on nil. Alternatively, they can be `.unsafelyUnwrapped()`, which will check and trap in debug builds of user code. Internal to the standard library is `._unsafelyUnwrappedUnchecked()` which will only check and trap in debug builds of the standard library itself. These correspond directly with `_precondition`, `_debugPrecondition`, and `_sanityCheck`. See [that section](#precondition) for details.
+Optionals can be unwrapped with `!`, which triggers a trap on nil. Alternatively, they can be `.unsafelyUnwrapped()`, which will check and trap in debug builds of user code. Internal to the standard library is `._unsafelyUnwrappedUnchecked()` which will only check and trap in debug builds of the standard library itself. These correspond directly with `_precondition`, `_debugPrecondition`, and `_internalInvariant`. See [that section](#precondition) for details.
 
 #### UnsafeBitCast and Casting References