docs: fix typos in ServeDir, ServeFile, and conditional headers

[Isvane]

Motivation

This PR fixes minor typographical, grammatical, and formatting errors found across the file-serving modules and documentation comments.

Solution

• ServeFile: Fixed a typo in a test name (precompresed -> precompressed).
• Header Utils (LastModified / IfModifiedSince):
  • Fixed typo silentely -> silently.
  • Corrected typo where IfUnmodifiedSince documentation incorrectly referred to IfModifiedSince.
  • Capitalized standard doc comments for consistency.
• ServeDir:
  • Fixed grammar in try_call doc example (your adding -> you are adding).
  • Updated specialise to specialize to match standard US English naming conventions used elsewhere in the crate (Customize).

[jlizen]

Thanks!