docs: add Float64 -> Float

Author: G-Rath

docs/migrate-v2-to-v3.md

@@ -129,6 +129,40 @@ Shell command to find them all: `fgrep -rl github.com/urfave/cli/v2 *`
     }
     ```
 
+### cli.Float64Flag
+
+=== "v2"
+
+    ```go
+    cli.Float64Flag{
+            // ...
+    }
+    ```
+
+=== "v3"
+
+    ```go
+    cli.FloatFlag{
+            // ...
+    }
+    ```
+
+### cli.Context.Float64
+
+=== "v2"
+
+    ```go
+    // ctx is cli.Context
+    ctx.Float64("my-floating-flag")
+    ```
+
+=== "v3"
+
+    ```go
+    // cmd is cli.Command
+    cmd.Float("my-floating-flag")
+    ```
+
 ## Sources
 
 ### FilePath