fix typo proposals/README.md

Co-authored-by: Marc Cenac <547446+mrcnc@users.noreply.github.com>

Author: julienledem

proposals/README.md

@@ -1,6 +1,6 @@
 # Proposals
 
-This proposal process is intended for significantly impactfull additions to the Parquet spec. The goal is to facilitate those projects and help them being contributed to Parquet.
+This proposal process is intended for significantly impactful additions to the Parquet spec. The goal is to facilitate those projects and help them being contributed to Parquet.
 For example, changes that are not backward compatible like adding a new encoding or a new compression algorithm (older implementations can not read new files).
 This gives better visibility to those projects which require coordination in several implementations.
 Bug fixes, code only features or minor changes to the spec that can be ignored by older implementations can simply be filed as a github issue.