Fix incorrect migration instructions in backward-compatibility.md
RELNOTES: None.
@@ -844,7 +844,7 @@ For Starlark rules using C++ Make Variables:
```python
# Before
def _impl(ctx):
- strip = ctx.vars["STRIP"]
+ strip = ctx.var["STRIP"]
...
my_rule = rule(
@@ -855,7 +855,7 @@ my_rule = rule(
# After
def _impl(ctx):
- strip = ctx.vars["STRIP"]
+ strip = ctx.var["STRIP"]
...
my_rule = rule(
