Widen the automatic coercion heading to cover interpolation

The section documents both arithmetic coercion and string interpolation,
but the heading named only arithmetic, so the pointer from the top of the
page under-described where it led. Drop the qualifier and update the
in-page link.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Michael Harp <mike@mikeharp.com>

Author: miharp

docs/_openvox_8x/lang_typecasting.md

@@ -25,7 +25,7 @@ Converting never changes the original value. It produces a new value, leaving th
 alone.
 
 Two things do convert without being asked: arithmetic on a string that looks like a number, and string
-interpolation. Both are covered in [automatic coercion](#automatic-coercion-in-arithmetic) below.
+interpolation. Both are covered in [automatic coercion](#automatic-coercion) below.
 
 ## Calling a data type as a constructor
 
@@ -281,7 +281,7 @@ input that might not contain a number at all:
 notice(scanf('no digits', '%d')) # []
 ```
 
-## Automatic coercion in arithmetic
+## Automatic coercion
 
 Arithmetic operators can coerce a string to a number, but by default OpenVox treats doing so as an error: