Fix "an QWP" grammar in Javadoc comment

Replace "an QWP" with "a QWP" in QwpColumnDef Javadoc.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mtopolnik

core/src/main/java/io/questdb/client/cutlass/qwp/protocol/QwpColumnDef.java

@@ -28,7 +28,7 @@
 import static io.questdb.client.cutlass.qwp.protocol.QwpConstants.TYPE_CHAR;
 
 /**
- * Represents a column definition in an QWP v1 schema.
+ * Represents a column definition in a QWP v1 schema.
  * <p>
  * This class is immutable and safe for caching.
  */