doc: typo in properVAdd_iff_continuousVadd_ultrafilter_tendsto (#17008

)

Just a typo in the doc.

Mathlib/Topology/Algebra/ProperAction.lean

@@ -86,7 +86,7 @@ instance (priority := 100) ProperSMul.toContinuousSMul [ProperSMul G X] : Contin
 /-- A group `G` acts properly on a topological space `X` if and only if for all ultrafilters
 `𝒰` on `X × G`, if `𝒰` converges to `(x₁, x₂)` along the map `(g, x) ↦ (g • x, x)`,
 then there exists `g : G` such that `g • x₂ = x₁` and `𝒰.fst` converges to `g`. -/
-@[to_additive "A group acts `G` properly on a topological space `X` if and only if
+@[to_additive "A group `G` acts properly on a topological space `X` if and only if
 for all ultrafilters `𝒰` on `X`, if `𝒰` converges to `(x₁, x₂)`
 along the map `(g, x) ↦ (g • x, x)`, then there exists `g : G` such that `g • x₂ = x₁`
 and `𝒰.fst` converges to `g`."]