Docs: Fix non-existent SortOrderBuilder method in evolution.md example

**Apache Iceberg version**
main @ 49b89a8c5

**Query engine**
None (Java API)

**Please describe the bug**
The `replaceSortOrder` example in `docs/docs/evolution.md` (line 96) calls `.dec("category", NullOrder.NULL_FIRST)`. Neither exists in the API: `SortOrderBuilder` (`api/src/main/java/org/apache/iceberg/SortOrderBuilder.java`) only defines `desc(...)`, not `dec(...)`, and `NullOrder` (`api/src/main/java/org/apache/iceberg/NullOrder.java` line 22-23) only defines the constants `NULLS_FIRST`/`NULLS_LAST`, not `NULL_FIRST`. The example does not compile as written. The line directly above it (`.asc("id", NullOrder.NULLS_LAST)`) already uses the correct constant naming, so only the second line has the typo.

**Steps to reproduce**
Copy the example code block from `docs/docs/evolution.md` line 93-97 into a Java file and compile it: fails with "cannot find symbol" for both `dec` and `NULL_FIRST`.

**Additional context**
N/A.

Open docs/docs/evolution.md around line 96 and compare the replaceSortOrder example with api/src/main/java/org/apache/iceberg/SortOrderBuilder.java and NullOrder.java. Verify the corrected example against the API, then confirm the code block no longer contains the reported compilation errors.

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