Update FlurlClientCache.cs

Tiny typo that I just noticed when migrating from 3.x and away from FlurlClientFactoryBase.
Apologies if I misunderstood and that's actually not a typo

src/Flurl.Http/Configuration/FlurlClientCache.cs

@@ -85,7 +85,7 @@ public virtual IFlurlClient Get(string name) {
 				throw new ArgumentException($"A client named '{name}' was not found. Either preconfigure the client using Add (typically at startup), or use GetOrAdd to add/configure one on demand when needed.");
 
 			if (cli.Value.IsDisposed)
-				throw new Exception($"A client named '{name}' was not found but has been disposed and cannot be reused.");
+				throw new Exception($"A client named '{name}' was found but has been disposed and cannot be reused.");
 
 			return cli.Value;
 		}