minor symfony#16903 Fix wrong method name mapping in UPGRADE-3.0.md (mheki)

This PR was merged into the 2.8 branch.

Discussion
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Fix wrong method name mapping in UPGRADE-3.0.md

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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2765088 Fix wrong method name mapping in UPGRADE-3.0.md

Author: fabpot

UPGRADE-3.0.md

@@ -11,7 +11,7 @@ UPGRADE FROM 2.x to 3.0
    | `registerNamespaces()` | `addPrefixes()`
    | `registerPrefixes()` | `addPrefixes()`
    | `registerNamespaces()` | `addPrefix()`
-   | `registerPrefixes()` | `addPrefix()`
+   | `registerPrefix()` | `addPrefix()`
    | `getNamespaces()` | `getPrefixes()`
    | `getNamespaceFallbacks()` | `getFallbackDirs()`
    | `getPrefixFallbacks()` | `getFallbackDirs()`