[docs] Fix stale paimon-vindex version in vector index documentation

[thswlsqls]

Purpose

• In docs/docs/multimodal-table/global-index/vector.mdx, line 126 names the paimon-vindex 0.3.0 identifiers while line 127 of the same sentence says to install paimon-vindex==0.4.0.
• [paimon-vector] Upgrade paimon-vector-index to 0.4.0 #9341 (193723d19, "[paimon-vector] Upgrade paimon-vector-index to 0.4.0") updated the install pin on line 127 but missed line 126.
• 0.4.0 is what the build resolves: paimon-vector/pom.xml sets paimon-vector-index-java.version to 0.4.0, and paimon-python/setup.py pins paimon-vindex==0.4.0.
• Scope: line 126 only. The 0.2.x -> 0.3.0 upgrade warning at L56-58 is a historical note and is left unchanged. The listed identifiers match the module factory constants and are untouched.

Tests

• Documentation-only wording change with no behavior change, so it carries no tests and no Maven build gate applies.
• Only a version number inside prose changes; the inline code backticks and line wrapping are unchanged, so the mdx rendering is unaffected.