bc: shrink internal library yet more

function                                             old     new   delta
bc_lib                                              1609    1604      -5
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-5)               Total: -5 bytes
   text	   data	    bss	    dec	    hex	filename
 980165	    485	   7296	 987946	  f132a	busybox_old
 980160	    485	   7296	 987941	  f1325	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/miscutils/bc.c b/miscutils/bc.c
index 8befc8e..dd2d7f2 100644
--- a/miscutils/bc.c
+++ b/miscutils/bc.c
@@ -7302,7 +7302,7 @@
 "\n"		"a=(x^n)/2^n/a"
 "\n"		"r=v=1"
 "\n"		"f=-x*x/4"
-"\n"		"scale=scale+length(a)-scale(a)"
+"\n"		"scale+=length(a)-scale(a)"
 "\n"		"for(i=1;v;++i){"
 "\n"			"v=v*f/i/(n+i)"
 "\n"			"r+=v"