Fix documentation for test-mass noise

lisainstrument/noises.py

@@ -253,7 +253,7 @@ def testmass(fs, size, asd, fknee):
     Multiplying by 1 / (2π f)^2 yields the noise as a velocity,
 
         S_delta(f) [m/s] = (asd / 2π)^2 [ f^(-2) + (fknee^2 / f^4) ]
-                         = (asd / 2π)^2 f^(-2) + (2 asd fknee / 2π)^2 f^(-4).
+                         = (asd / 2π)^2 f^(-2) + (asd fknee / 2π)^2 f^(-4).
 
     Args:
         asd: amplitude spectral density [ms^(-2)/sqrt(Hz)]